Do not delete old SkyModelPre attributes until we know if new ones ar…

…e valid

rubin_sim/skybrightness_pre/sky_model_pre.py

@@ -155,9 +155,6 @@ def _load_data(self, mjd, filename=None, npyfile=None):
         npyfile : `str` (None)
             If sky brightness data not in npz file, checks the .npy file with same root name.
         """
-        del self.sb
-        del self.filter_names
-        del self.timestep_max
 
         if filename is None:
             # Figure out which file to load.
@@ -174,6 +171,23 @@ def _load_data(self, mjd, filename=None, npyfile=None):
         else:
             self.loaded_range = None
 
+        # Use three separate try/excepet blocks so that if any of them throw
+        # exceptions, we still get the others.
+        try:
+            del self.sb
+        except AttributeError:
+            pass
+
+        try:
+            del self.filter_names
+        except AttributeError:
+            pass
+
+        try:
+            del self.timestep_max
+        except AttributeError:
+            pass
+
         if self.verbose:
             print("Loading file %s" % filename)
         h5 = h5py.File(filename, "r")